Cogito ergo sum

Descartes' axiom. This is a petitio principii. “I think”
can only prove this: that “I think.” And he might just as well infer
from it the existence of thought as the existence of I.
He is asked to prove the latter, and immediately assumes that it exists
and does something, and then infers that it exists because it
does something. Suppose I were asked to prove the existence of ice, and
were to say, ice is cold, therefore there is such a thing as ice.
Manifestly I first assume there is such a thing as ice, then ascribe to
it an attribute, and then argue back that this attribute is the outcome
of ice. This is not proof, but simply arguing in a circle.